CVE-2026-126393
Deferred Deferred - Pending Action

SiYuan SSRF Vulnerability in Skill-Install Download Path

Publication date: 2026-11-13

Last updated on: 2026-11-13

Assigner: VulnCheck

Description
SiYuan before 3.8.10 contains a server-side request forgery vulnerability in the skill-install download path because fetchBytes performs only a parse-time hostname check instead of using ssrfSafeClient. Attackers can supply a skill URL via the MCP skill install action that redirects or DNS-rebinds to loopback addresses, reaching internal services and metadata endpoints.

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Meta Information

Published
2026-11-13
Last Modified
2026-11-13
Generated
2026-11-13
AI Q&A
2026-11-13
EPSS Evaluated
N/A
NVD
CVE-2026-126393
EUVD
EUVD-2026-125172

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
siyuan siyuan to 3.8.10 (exc)

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-918 The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

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Executive Summary

SiYuan before 3.8.10 has a server-side request forgery (SSRF) vulnerability in the skill-install download path. The fetchBytes function only checks hostnames at parse time and uses a plain HTTP client that resolves hostnames again at connection time. This allows attackers to supply a skill URL that redirects or DNS-binds to loopback addresses (e.g., 127.0.0.1), bypassing SSRF protections and accessing internal services or metadata endpoints.

Detection Guidance

To detect this vulnerability, monitor network traffic for outbound requests from SiYuan to internal or loopback addresses (e.g., 127.0.0.1, 192.168.x.x). Check SiYuan logs for skill-install actions that involve redirects to unexpected destinations. Use tools like tcpdump or Wireshark to capture HTTP requests from the SiYuan process.

Impact Analysis

An attacker could exploit this to access internal services on your machine, such as local databases or APIs, by tricking you into installing a malicious skill. This requires minimal user interaction, like clicking a link, and could lead to data theft or unauthorized actions on internal systems.

Compliance Impact

This SSRF vulnerability could lead to unauthorized access to internal systems, potentially exposing sensitive data. This may violate GDPR's data protection requirements or HIPAA's safeguards for protected health information, depending on the data accessed. Compliance could be impacted if unauthorized data access or breaches occur due to the vulnerability.

Mitigation Strategies

Upgrade SiYuan to version 3.8.10 or later to address the SSRF vulnerability. If upgrading is not immediately possible, restrict network access for the SiYuan application using firewall rules to block outbound connections to internal addresses. Disable the MCP skill install feature if unused.

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